Voltage Controlled Oscillators (VCOs) constructed of resonant circuits are used in communication devices as the means of generating a desired frequency of operation. Recent VCO designs don't use varactors which were the traditional means of tuning oscillators. The elimination of varactors in VCOs results in significant benefits in monolithic applications were an entire circuit is realized on a single Integrated Circuit (IC). One such design is shown in FIG. I and fully described in an article titled "A 1.8 GHz Monolithic LC Voltage Controlled Oscillator" written by Nyugen and Meyer and published in the IEEE Journal of Solid State Circuits. VOL. 27, No. 3, March 1992.
Referring to FIG. 1, it can be seen that this design includes a tunable oscillator constructed without varactors from a Gilbert multiplier cell and two tank circuits. Bipolar transistors and current sources comprise the Gilbert cell. A problem with this design architecture is its limitation at low supply voltages. There are two factors that prevent effective operation at low DC voltages. First, it is necessary to bias transistors 102-112 such that operation outside of the saturation region is maintained. The current source 114 is typically constructed of a bipolar transistor and thus requires a similar restriction. A typical set of bias conditions is shown in FIG. 1. The tuning lines at the base of transistors 102-108 are assumed to be 1.7 V and V.sub.BE is 0.9 V. Under this set of conditions the voltages at the base nodes of transistors 110 and 112 are at a steady state value of 1.2 V while the collectors are at 0.8 V. Assuming a collector-emitter saturation voltage of 0.3 V, an AC signal of only 0.2 V applied to the bases of transistors 110 and 112 would place the devices in saturation. Similarly, transistors 102 and 108 are driven into saturation by a 0.7 V.sub.AC signals applied to their collectors.
A second problem with the circuit of prior art is that the tuning voltage is restricted to the range of approximately .+-.0.2 V. This is a result of the differential-pair structures which control the tuning operation. A low tuning voltage range results in high sensitivity to noise at the tuning ports 101 and 109 which results in poor noise performance of the oscillator. The typical method for increasing the linear range of a differential amplifier, adding resistance in the emitter branches of the circuit, can not be applied in low voltage circuits because it exacerbates the problem with device saturation.
These problems prevent the effective use of this circuit in portable radio applications which operate using batteries. It is desired to have a "varactorless" VCO that overcomes the deficiencies of the prior art.